In general, moving image data encoding methods are roughly classified based on whether or not to use inter-frame correlation. Those methods have merits and demerits, and which of methods is suitable depends on an application used. For example, Motion JPEG is a method of independently encoding each frame of moving image data as a single still image, and is an example of an encoding method that does not use any inter-frame correlation. As merits, this method allows easy moving image edit processes such as a division process, coupling process, partial rewrite process, and the like of moving images, and allows to select and decode frames to be decoded in accordance with the processing performance on the decoding side, since frames are independently encoded.
In recent years, of the encoding methods that independently encode moving image data for respective frames, a method of encoding each frame by combining wavelet transformation and bitplane encoding has received a lot of attention. Such moving image encoding method has outstanding features: it allows decoding by changing the spatial resolution step by step by exploiting a subband decomposition scheme in wavelet transformation, and can change the decoding pixel precision step by step by changing the number of bitplanes to be decoded.
JPEG2000 (ISO/IEC 15444) as an image encoding method that has been standardized by ISO/IEC JTC/SC29/WG1 is specified by a combination of wavelet transformation and bitplane encoding. Part 3 of that standard specifies a file format as Motion JPEG2000 upon applying JPEG2000 to encoding of respective frames of a moving image.
Such moving image encoding methods represented by Motion JPEG2000 have merits such as flexibility in decoding resolution and decoding pixel precision, as described above, but have demerits such as a heavy load on an encoding/decoding process by bitplane encoding. Especially, when a video recorded by a dedicated moving image recording device is to be played back by a personal computer, all data cannot often be decoded and displayed in real time depending on the performance of the computer.
To solve such problem, methods that determine a decoding process time required to decode each frame, assign the decoding process time for respective encoding process units, and decode respective bitplanes within the assigned decoding process time have been disclosed (e.g., Japanese Patent Laid-Open Nos. 11-288307 and 2001-112004).
However, with a moving image decoding apparatus which aborts a decoding process after an elapse of a predetermined time, as disclosed in Japanese Patent Laid-Open Nos. 11-288307, 2001-112004, and the like, since the decoding process abort points (the number of decoded bitplanes) readily vary for respective frames, a change in distortion pattern over time occurs upon playing back a moving image, and appears as flickering, thus causing visual disturbance.